test/extended/networking: fix [pod sysctls should not affect node] to use host interfaces
test/extended/networking: fix [pod sysctls should not affect node] to use host interfaces

This test was listing all interfaces on the host with 'ls /sys/class/net'
which does not return a stable order of interfaces. Thus it was sometimes
reading host-side veths of pods, which would be gone (because we run
tests in parallel and pods are starting/stopping all the time) by the time
the test wanted to read the interface's sysctls.

Jun  2 16:13:38.827: INFO: Error running /usr/bin/oc --namespace=e2e-test-tuning-vz9kr --kubeconfig=/tmp/secret/kubeconfig exec nodeaccess-pod-lmbdb -- cat /host/proc/sys/net/ipv4/conf/090399bc34a8bcf/send_redirects:
StdOut>
cat: can't open '/host/proc/sys/net/ipv4/conf/090399bc34a8bcf/send_redirects': No such file or directory
command terminated with exit code 1
StdErr>
cat: can't open '/host/proc/sys/net/ipv4/conf/090399bc34a8bcf/send_redirects': No such file or directory
command terminated with exit code 1

Instead of grabbing random interfaces, grab "/sbin/ip link show" which will
be in sorted order, and ensures we get both the interface name and index
at the same time. Pick the first non-loopback interface, which is almost
certianly a "real" host interface.

mfojtik/origin,mfojtik/origin,csrwng/origin,jsafrane/origin,akram/origin,openshift/origin,csrwng/origin,deads2k/origin,openshift/origin,hroyrh/origin,mahak/origin,deads2k/origin,bparees/origin,soltysh/origin,mfojtik/origin,hroyrh/origin,akram/origin,bparees/origin,soltysh/origin,akram/origin,mahak/origin,openshift/origin,bparees/origin,jsafrane/origin,jsafrane/origin,csrwng/origin,mahak/origin,soltysh/origin,deads2k/origin,hroyrh/origin